1. Field of the Invention
The present invention relates to a fluorescent X-ray analysis apparatus which performs an element analysis and a composition analysis of a sample by irradiating a primary X-ray to the sample and detecting a fluorescent X-ray generating from the sample.
2. Description of the Related Art
In recent years, a cadmium pollution of a food, and the like become a problem, and a quantitative determination of a cadmium content in the food, and the like are performed. Hitherto, in the quantitative determination of cadmium, although there have been performed an ICP (inductively coupled plasma spectrometry) and the like, there have been problems that, in addition to the fact that a time is necessary for such a pretreatment as to make the sample into a solution, a dispersion occurs in a measurement result in dependence on an operator. From the background like this, as a measurement method substituted for the ICP, a fluorescent X-ray analysis is noted. The fluorescent X-ray analysis is one in which a kind and a quantity of an element contained in the sample is specified by irradiating the primary X-ray to the sample and detecting the generated fluorescent X-ray, and hitherto it has been utilized mainly in an analysis of the sample, such as Cu alloy or Fe alloy, whose main component is a heavy element, and the like. Since the fluorescent X-ray has an intensity and an energy which are inherent to the element, it is possible to specify the element having been contained in the sample and its quantity by detecting an intensity and an energy of the generated fluorescent X-ray. In the fluorescent X-ray analysis, there suffices if the primary X-ray is directly irradiated to the sample, and there are advantages that a measurement is possible even if the sample is not pretreated and, also as to an analysis result, a reproducibility is good in comparison with the ICP. A detection lower limit denoting an accuracy of the fluorescent X-ray analysis like this is determined by the following expression.Detection lower limit=3×(√Background intensity/Measurement time)/Sensitivity
Here, the background intensity means mainly an intensity of a scattered X-ray or the like other than the fluorescent X-ray generating from an aimed element having been contained in the sample. Further, the sensitivity is a magnitude of an X-ray intensity obtainable in a detector. That is, by decreasing the background intensity and further raising the sensitivity, the detection lower limit is improved, and it becomes possible to realize the quantitative determination of a trace element.
As the fluorescent X-ray analysis apparatus capable of performing the fluorescent X-ray analysis like this, for example, there is proposed one having possessed an X-ray source irradiating the primary X-ray to the sample, a detector detecting the fluorescent X-ray generated from the sample to which the primary X-ray has been irradiated, and a primary filter having plural filter components, or the like (e.g., refer to JP-A-2004-150990 Gazette). According to the fluorescent X-ray analysis apparatus like this, by absorbing the primary X-ray of plural energy bands by the primary filter and irradiating the primary X-ray of a necessary energy band, it is possible to decrease the background intensity, thereby improving the detection lower limit.
However, the primary X-ray having been irradiated to the sample excites the sample to thereby generate the fluorescent X-ray (primary fluorescent X-ray) and, by the sample, scatters to a periphery as a primary scattered ray. And, between the primary fluorescent X-ray and the primary scattered ray, one part having been not detected by the detector generates, by the fact that it is irradiated to an X-ray source, an outer periphery face of the detector or the like, a secondary X-ray. That is, by the fact that it scatters in the X-ray source, the outer periphery face of the detector or the like, a secondary scattered ray generates and, further by the fact that it excites elements forming the X-ray source, the outer periphery face of the detector or the like, a secondary fluorescent X-ray generates. And, one part of the X-ray having been generated secondarily scatters directly or again in the sample and is detected by the detector. That is, by the fact that the unnecessary X-ray having generated subsidiarily, which is the X-ray other than the primary fluorescent X-ray to be detected originally, is detected by the detector, a count (intensity) of the X-ray entering into the detector increases. In a case like this, since there is a limit in the count of the X-ray capable of being detected by the detector, although it is necessary to suppress the intensity of the primary X-ray irradiated from the X-ray source, the intensity of the primary fluorescent X-ray capable of being detected lowers as well, so that there has been a problem the detection lower limit deteriorates as a result. Further, by placing a member (hereafter, called a collimator), which has a through-hole, in a front face of the detector, although it is possible to suppress the count of the X-ray entering into the detector, since the secondary fluorescent X-ray generating from a hole wall of the through-hole of the collimator is detected in its most by the detector, the count of the X-ray is increased by this X-ray generating secondarily, so that it is impossible to fundamentally reduce the X-ray other than the primary fluorescent X-ray to be detected originally.
Further, in a count circuit, by the fact that the count increases in such a degree that the X-rays having generated subsidiarily cannot be discriminated as separate ones, a count error (hereafter, called a pileup) occurs. The pileup exerts two adverse effects on a spectrum obtainable. One is a deterioration (a peak width of the spectrum becomes thick) of an energy resolving power. The other one is the fact that a pseudo-peak called “sum-peak” is formed. Both increase the background intensity, thereby deteriorating the detection lower limit. From the problem like this, as mentioned above, although there is noted the quantitative determination of the trace aimed element such as the cadmium content in the food, due to these X-rays generating subsidiarily, there has not led to obtain the detection lower limit under which the quantitative determination of the trace aimed element is possible.